SpecularMaskMaterial added

Added support for specular masking through the use of Specular texture
added in TextureType. Texture mask is a grayscale image where black
pixel allows for full specularity and white pixel completely masks out
specularity.
Fix inverted specular maps

Fixed inverted specular maps to follow a more common convention.
White = transparent
Black = opaque
Merge pull request #306 from AndrewJo/master

Specular masking support added